The present invention relates to an FM stereo demodulation circuit.
An analog multiplication of a composite signal and a sinusoidal signal having a subcarrier frequency of 38 kHz is generally preferred for FM stereo demodulation. However, it is difficult to obtain an ideally linear analog multiplier. For this reason, in general, a switching system as shown in FIG. 1 utilizing a rectangular switching waveform is used. Referring to FIG. 1, a composite signal v(t) is supplied to switching elements TR1 and TR2. The switching elements TR1 and TR2 are alternately turned on by a 38 kHz switching signals S(t) and S(t) from an oscillation circuit OSC (usually a phase-locked loop circuit is used) which are synchronized with a pilot signal of 19 kHz in the composite signal. The switching signals S(t) and S(t) are opposite in phase to each other. The composite signal v(t) is multiplied by the switching signal of 38 kHz. Left and right audio signals are taken from low-pass filters F1 and F2, respectively.
A rectangular wave having a duty ratio of 50% as shown in FIG. 2 is generally used as the switching signal S(t). In this case, a demodulation output is given by ##EQU1##
As is apparent from the above equation, when the composite signal v(t) includes noise components of frequencies 3.omega., 5.omega. and so on, the demodulation circuit has demodulation sensitivity for frequency components of 3.omega., 5.omega., etc. In this manner, when the rectangular waveform having duty ratio of 50% is used, the demodulation circuit has great demodulation sensitivity for input signals of 114 kHz (38 kHz.times.3), 190 kHz (38 kHz.times.5) or the like, as shown in FIG. 3. The demodulation sensitivity for the 3.omega. component is only 10 dB lower than the demodulation sensitivity for the .omega. component. For this reason, when the 3.omega. and 5.omega. frequency noise components are included in an output signal of an FM detector, S/N ratio of the output signal of the FM stereo demodulation circuit decreases and beat interference occurs. In order to solve these problems, a filter may occasionally be provided before the demodulation circuit to eliminate the 3.omega., 5.omega. . . . frequency components. With this approach, however, it is difficult to ensure the flatness of the amplitude and phase characteristics of the composite signal which has a frequency range of 0 to 53 kHz, thus degrading the frequency characteristic of stereo separation.